The present invention relates to magnetic powder which is the main component of a bonded magnet used in a motor and the like in large quantities, a method for producing mother alloy powder thereof, and a high pressure heat-treatment apparatus for use in these methods.
A high efficient bonded magnet using high efficient magnetic powder containing rare earth elements as a main component such as anisotropic magnetic powder of the Sm-Co system and isotropic magnetic powder of the Nd-Fe-B system is utilized in devices applying a magnet such as a motor. Such a bonded magnet has come to find wider application gradually. With this trend, there has been required magnets having various characteristics.
Under such circumstances, new magnetic powders have been intensively developed. Among them, magnetic materials of the rare earth(R)-iron(Fe)-nitrogen(N) system, especially magnetic materials of samarium (Sm)-iron-nitrogen system (hereinafter, referred to as Sm-Fe-N system) have attracted attention. However, the development of the magnetic materials of the Sm-Fe-N system is still in the laboratory stage, and hence there has been demand for practical utilization thereof as soon as possible.
The magnetic materials of the Sm-Fe-N system can be obtained by nitrogenating Sm.sub.2 Fe.sub.17 with a structure of Th.sub.2 Zn.sub.17, in which nitrogen atoms enter the specified interstitial sites of the crystal lattice to expand the crystal lattice, resulting in the same crystal structure of the Th.sub.2 Zn.sub.17. The magnetic materials of the Sm-Fe-N system exhibit the most excellent magnetic characteristics when the value of X is in the neighborhood of 3 in a composition formula of Sm.sub.2 Fe.sub.17 N.sub.x. The following basic physical properties have been revealed: saturation magnetization: 4.pi.I.sub.S =15.7 kG, anisotropic magnetic field: Ha=260 kOe, and Curie point: Tc=470.degree. C.
The processes of producing the magnetic materials of the Sm-Fe-N system can be roughly classified into the following 5 processes (See, ex., Japanese Laid-Open Patent Publication No.2-57663):
(1) Melting and casting process: a process for producing a Sm-Fe mother alloy ingot; PA1 (2) Homogenizing heat-treatment process: a process for making the Sm-Fe mother alloy ingot into the alloy ingot with a main phase of Sm.sub.2 Fe.sub.17 ; PA1 (3) Coarse crushing process: a process for crushing the alloy ingot with a main phase of Sm.sub.2 Fe.sub.17 into particles with a size of 150 .mu.m or less which is easily nitrogenated; PA1 (4) Nitrogenating process: a process for making Sm.sub.2 Fe.sub.17 into Sm.sub.2 Fe.sub.17 N.sub.x ; and PA1 (5) Fine crushing process: a process for making the Sm.sub.2 Fe.sub.17 N.sub.x particles into magnetically single domain particles (particle size: 1 to 3 .mu.m) in order to enhance the coercive force thereof.
In addition to the above-described processes, annealing treatment is conducted when required after the process (3) or (5) so that distortion, lattice defect or the like resulting from the size reduction step is eliminated, and after the process (4) so that ununiform nitrogenated layer is made uniform, respectively.
Below, each of the above-described processes will now be described in more detail.
In the coarse crushing process (3), crushing has been performed mechanically in conventional methods. For example, various crushers such as jaw crusher have been employed. Also, it has been reported that there is a possibility of coarse crushing by hydrogen decrepitation employed in the production process of sintered magnet of the Nd-Fe-B system.
This hydrogen decrepitation is a conventional method in which the absorption and desorption of hydrogen are repeatedly performed. That is, hydrogen gas is absorbed into alloy at a temperature in the range of 200.degree. to 400.degree. C., after which the absorbed hydrogen is desorbed in an inert gas such as argon at a temperature in the range of 600.degree. to 800.degree. C. It has been reported that an increase in the number of this repetition enables the coarse crushing down to 4 .mu.m (See, Japanese Laid-Open Patent Publication No.2-57663 and EP0369097A1).
Besides, hydrogen decrepitation of Sm.sub.2 Fe.sub.17 has been studied and reported in Tohoku University in recent years (S. Sugimoto et al.: Ferrite Proceedings of the ICF6, Tokyo and Kyoto, Japan, 1992, pp.1145 to 1148). According to this paper, Sm.sub.2 Fe.sub.17 subjected to homogenizing heat-treatment will not undergo hydrogen decrepitation in the temperature range of room temperature to 200.degree. C., while it will undergo hydrogen decrepitation at temperatures in the neighborhood of 300.degree. C., resulting in not fine powder, but coarse powder (with a mean particle size of 2 to 3 mm).
However, in the composition wherein the proportion of Sm is increased (by 20% or more), in addition to Sm.sub.2 Fe.sub.17, SmFe.sub.3 appears after homogenizing heat-treatment. This SmFe.sub.3 causes sharp hydrogen absorption at 200.degree. C., and also the volume expansion of the crystal lattice of SmFe.sub.3 is larger than that of Sm.sub.2 Fe.sub.17. Therefore, the alloy ingot is broken into fine powder (with a particle size of 100 .mu.m or less).
Also, the result of the measurement of hydrogen absorption and desorption characteristics showed that Sm.sub.2 Fe.sub.17 exhibited a peak of hydrogen absorption at temperatures in the neighborhood of 250.degree. C. and 600.degree. C., between which, i.e., in the temperature range of 350.degree. C. to 550.degree. C., it exhibited a gentle peak of hydrogen desorption.
There has been another report on hydrogen absorption heat-treatment. With magnetic materials made of 5 to 15 atomic % Sm, 0.5 to 25 atomic % N, and Fe or Fe and Co in the remaining part, a fine particle of Sm-Fe alloy is readily oxidized. Therefore, a method is disclosed in the report whereby alloy in a large lump form, i.e., a large alloy ingot in which a region at a distance of 0.25 mm or more from the surface exists can be nitrogenated. In this method, an alloy ingot is subjected to hydrogen absorption heat-treatment to be allowed to absorb hydrogen, after which the alloy ingot is subjected to nitrogenating heat-treatment in an atmosphere of nitrogen gas, thus enabling alloy of a large size to be fully and uniformly nitrogenated. The above-described report explains that fine passage for gas is formed in alloy in hydrogen absorption heat-treatment so that nitrogen enters to the deep portion in the alloy through the resulting gas passage in nitrogenating heat-treatment. Also, as disclosed in the report, in this hydrogen absorption heat-treatment, the temperature is 350.degree. C. or less, and especially preferable in the range of 100.degree. to 300.degree. C. It is also important that hydrogen absorption heat-treatment is not conducted twice or more. When hydrogen absorption heat-treatment is conducted only one time, alloy is not reduced to fine powder, remaining of a large size (See, Japanese Laid-Open Patent Publication No.4-280605).
As a nitrogenating process (4), conducted are a method whereby Sm.sub.2 Fe.sub.17 is exposed to a stream of hydrogen-ammonia gas mixture at temperatures in the neighborhood of 470.degree. C. (Japanese Laid-Open Patent Publication No.2-257603); and a method whereby alloy is heat-treated in nitrogen gas at the same temperatures and high pressure (of 30 kgf/cm.sup.2 or more) (Tatami et al., Digests of the 16th Annual Conference on Magnetics in Japan, 1992, p440, and Japanese Laid-Open Patent Publication No.5-258927). Also, there has been another report as follows: Sm.sub.2 Fe.sub.17 powder coarsely crushed to 53 .mu.m or less is placed in a stream of hydrogen gas at temperatures in the neighborhood of 250.degree. C. for 1 hour to be subjected to hydrogen treatment, after which the powder is placed in a stream of nitrogen gas to be heat-treated at low temperatures (of 450.degree. to 500.degree. C.) for long time (of 20 to 63 hours), thereby inhibiting the decomposition into .alpha.-Fe to obtain excellent magnetic characteristics (C. Ishizaka et al., Ferrite: Proceedings of The ICF 6, Tokyo and Kyoto, Japan 1992, pp.1092 to 1095).
As a fine crushing process (5), conventional methods of size reduction by a ball mill or jet mill are performed. According to the above-described paper by C. Ishizaka, the size reduction by means of an attritor provided fine powder with a particle size of 1.5 .mu.m or less, thus achieving excellent magnetic characteristics.
However, Sm-Fe alloy is readily oxidized, and oxidization thereof results in not only a decrease in the amount of Sm.sub.2 Fe.sub.17 N.sub.x which exhibits magnetic characteristics but also appearance of .alpha.-Fe. This .alpha.-Fe exhibits soft magnetism, and hence becomes the main cause of a decrease in its coercive force as well as deterioration in the rectangularity in hysteresis loop of a 4.pi.I-H curve. Therefore, control on oxidation has become a main problem of determining the quality of magnetic characteristics.
In order to control the oxidation of Sm-Fe alloy, the following methods can be considered.
The melting and casting process (1) and homogenizing heat-treatment process (2) are conducted in the active temperature range in excess of 1000.degree. C. Therefore, the oxygen concentration must be controlled as low as possible. However, since it is difficult to remove oxygen in an atmosphere completely, and also difficult to prevent the atmosphere in the time-consuming homogenizing heat-treatment process from including a small amount of oxygen due to cost and restrictions on facilities, the surface of the obtained alloy ingot is often oxidized.
The coarse crushing process (3) is performed at room temperature, but it can be considered that the temperature becomes considerably high at the scene of crystal break microscopically, and therefore it is preferably performed in an atmosphere of non-oxidizing gas (in many cases nitrogen gas).
The nitrogenating process (4) is performed in the temperature range of 400.degree. C. to 500.degree. C., but nitrogenate is decomposed into .alpha.-Fe and SmN at temperatures of about 670.degree. C. or more. Since nitrogenating is an exothermic reaction, it is necessary that the nitrogenating temperature is controlled low so as not to reach the decomposition temperature. On the other hand, since the nitrogenating rate increases with an increase in temperature, there has been a demand for conducting a nitrogenating process at temperatures as high as possible. According to the announcement in academy so far, nitrogenating is performed at about 470.degree. C. in most cases, and it can be said that this temperature is the optimum temperature for minimizing the generation of .alpha.-Fe.
However, the time as long as the order of 100 hours is required for completely nitrogenating Sm-Fe alloy at the above-described nitrogenating temperature. Also, the above-described nitrogenating temperature is too low for nitrogen attached to the alloy surface to diffuse into the interior of the Sm-Fe alloy, resulting in low diffusion rate. Reduction in particle size can shorten the nitrogenating time as well as conduct nitrogenating uniformly. Many reports have indicated that the size of particle is 50 .mu.m or less, preferably 20 .mu.m or less.
However, when the particle size of Sm-Fe alloy decreases, the surface area of the particle significantly increases, which causes the alloy to become markedly susceptible to oxidation. Accordingly, the inclusion of the oxygen in a nitrogenating process must be controlled as little as possible.
The fine crushing process (5) is a process for reducing the powder to fine powder with a particle size in the range of 1 to 3 .mu.m, and hence the surface area of the particle significantly expands, making the particle more susceptible to oxidation. Although the treating temperature is room temperature, it can be considered that the temperature at the scene of crystal break becomes high microscopically, requiring the fine crushing process to be performed in a non-oxydizing atmosphere.
From the above-described reasons, in order to prevent Sm-Fe alloy from being oxidized as well as remove .alpha.-Fe resulting from oxidation, provided are the following means, each of which has problems as described below.
First, in the homogenizing heat-treatment process (2), it is impossible to remove oxygen in an atmosphere completely as described above, and therefore Sm is oxidized at the surface of alloy ingot by all means. Also, since Sm has a high vapor pressure, Sm at the surface portion of alloy ingot vaporizes, which combines with the above-described oxidation to cause a decrease in the content of Sm at the surface portion of alloy ingot. This causes excess Fe to appear as .alpha.-Fe. This .alpha.-Fe degrades the magnetic characteristics and hence is required to be removed. Therefore, procedure such as shaving off the surface portion of alloy ingot was conducted in conventional processes, but this procedure is labor-intensive and time-consuming. Accordingly, there has been a demand for the development of efficient removal method.
The coarse crushing process (3) is performed with a crusher being placed in an atmosphere of nitrogen gas and the work is done through rubber gloves, resulting in inferior working efficiency. Also, the amount of nitrogen to be used is large, which leads to an increase in price of magnetic materials. Therefore, improvement thereof has been demanded. Also, mechanical crushing introduces many defects and impurities to the particle surface, contributing to deterioration in final magnetic characteristics.
Further, in order to transfer powder from the coarse crushing process to the subsequent nitrogenating process, once the coarsely crushed powder is taken out in air, and then put into a nitrogenating furnace. In this step, it is required that the inside of the furnace containing powder is evacuated and also the temperature is increased to temperatures in the neighborhood of 200.degree. C. to perform baking of powder and the inside of the furnace, thereby removing molecules of oxygen and water attached to the particle surface and the inwall of the furnace.
In the nitrogenating process (4), nitrogenating by ammonia gas entails the following problems. The gas is harmful, and although nitrogenating can be performed for a short time, only the surface portion tends to be excessively nitrogenated, resulting in deterioration in magnetic characteristics. Also, nitrogenating by nitrogen gas will not lead to excessive nitrogenating, but it is time-consuming.
In the fine crushing process (5), the use of a ball mill introduces a problem of requiring a time as long as 50 to 100 hours. With a jet mill, there occurs a problem that the amount of nitrogen gas to be used is large. The use of an attritor shortens the time required for the process, but there is still a problem that 3 hours are required under the optimum conditions.